A phantom power supply put virtually nothing in the signal path except a couple of electrolytic capacitors and even Neve have to have them!
Whatever you get, plug it in straight away and check mic function. Set for a good level of gain (hear clock ticking on cans ) then unplug the mic. The result should be silence. Obviously the supply cannot IMPROVE the system noise but it should not add to it. I have bought two different brands in the 'last year or so. One, a 'Chord' make was rubbish, could not deliver anything like enough current. The other name of which escapes me, was fine but both were commendably quiet.
Yes, Mark is on the money! Watch the mains voltage!
Thinking about things (As I am wont to do) Why does nobody AFAIK make a USB phantom power unit? There is 5V at 500mA (900 for USB 3.0) available and bus powered AIs generate it ok. Will pop an email to Messrs Orchid later!
Dave.